The invention relates to a method and an apparatus for monitoring the product quality of a production plant.
It is known to continuously monitor in non-destructive manner articles produced by a production plant. Use is made of eddy current, stray flux, ultrasonic, X-ray and other physical operating principles. Normally the testing systems generate analog test signals, which are displayed on monitors, so that the product quality can be read off. Normally a threshold value is given and on passing above or below the latter an alarm or some other fault indication is produced. Frequently the number of times limit values are exceeded are counted and used as a quality or fault measure. However, such known methods can only take account of whether or not a threshold value is exceeded. Thus, account cannot be taken of smaller signals which are below the fixed thresholds. Thus, no account is taken of part of the information content of the test signals. Thus, it is not possible to detect or follow e.g. a slow rise in the faults or defects prior to reaching the threshold value,. However, this behaviour would be of considerable significance if the aim is to describe future behaviour, e.g. state that shortly the production plant may operate defectively. For example it is not possible to recognize minor roughness differences of material surfaces or different chemical compositions only represented in the structure of the so-called parasitic signal.
An object of the invention is to provide a method of the aforementioned type, which permits a more sensitive and objective statement of the quality and changes to the quality of the product.